Methods of hardening cylinders or cylinder liners in which pistons of reciprocating internal combustion engines (ICE) operate have previously been proposed--see, for example, U.S. Pat. No. 4,093,842. The referenced publication describes the use of high-power carbon dioxide lasers to hardening cast iron.
Cylinders or cylinder liners or cylinder sleeves which have been hardened in accordance with prior art methods provide for hardening tracks which are close to each other or which overlap. Internal or inherent tension stresses will result in the region between two hardening tracks or in the overlap region. These stresses may be so great that, during operation of the ICE. fissures will occur in the walls of the cylinder or cylinder liner or sleeve. Such fissures may have a length of up to several centimeters. These fissures occur after hardening and subsequent honing of the cylinder walls or the liners or sleeves.
The method of hardening the cylinder surfaces or the surfaces of the liners or sleeves is essentially this: The cylinders or the liners are made of cast iron which is low in phosphor.
(a) The cylinder bore is first prepared for subsequent hardening by metal removal operations, for example on a lathe or boring machine, followed by honing. The metal removal processes are carried out until the cylinder bore, in the region to be hardened, has a diameter which is preferably about 0.02 to 0.05 mm smaller than the eventually desired final diameter. The surface of the cylinder bore will have a roughness RZ 0.015 mm.+-.0.003 mm.
(b) As the next step, an absorption substance is applied to the surface to be hardened, which will lower the reflection of laser light to a few percent.
(c) The third step in the process is the hardening of the cylinder bore in the region under question, by means of a laser beam, such that hardening tracks with martensitic structure in the edge zone of the cast iron will result.
A typical hardening laser is a 5 kw carbon dioxide laser. The laser beams are so guided with respect to the wall of the cylinder bore or the wall of the liner that parallel, helically progressing hardening tracks will result. This is achieved by rotating the cylinder or the cylinder bore relatively to the laser while, additionally, providing for longitudinal feed of the cylinder or cylinder bore or of the laser, respectively. The longitudinal feed and the speed of rotation are so matched that the pitch of the helical tracks of the laser beam will have the appropriate value to place the laser beam tracks where desired.
The laser beam is preferably formed in an integrator which provides a hardening track of essentially rectangular cross section and a uniform distribution profile of beam intensity for hardening. The hardening depth can be controlled and, preferably, is between about 0.5 mm to 1.3 mm.